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For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial From: Louis Lefebvre, Primate encephalization. In Michel A. Hofman, Dean Falk, editors: Progress in Brain Research, Vol. 195, Amsterdam: The Netherlands, 2012, pp. 393-412. ISBN: 978-0-444-53860-4 © Copyright 2012 Elsevier B.V. Elsevier Author's personal copy M. A. Hofman and D. Falk (Eds.) Progress in Brain Research, Vol. 195 ISSN: 0079-6123 Copyright Ó 2012 Elsevier B.V. All rights reserved. CHAPTER 19 Primate encephalization Louis Lefebvre* Department of Biology, McGill University, Montréal, QC, Canada Abstract: Encephalization is a concept that implies an increase in brain or neocortex size relative to body size, size of lower brain areas, and/or evolutionary time. Here, I review 26 large-scale comparative studies that provide robust evidence for five lifestyle correlates of encephalization (group living, a large home range, a high-quality diet, a strong reliance on vision, arboreal and forest dwelling), six cognitive correlates (better performance in captive tests, more tactical deception, innovation, tool use, social learning, all subsumed in part by general intelligence), one life history correlate (a longer lifespan), two evolutionary correlates (a high rate of change in microcephaly genes, an increase in brain size over macroevolutionary time), as well as three trade-offs (a slower juvenile development, a higher metabolic rate, sexually selected dimorphism). Of the 26 different encephalization measures used in these studies, corrected neocortex size, either with a ratio or a residual, is the most popular structural correlate of the functional variables, while residual brain size is the measure associated with the greatest number of them. Controversies remain on corrected or absolute measures of neural structure size, concerted versus mosaic evolution of brain parts and specialized versus domain-general brain structures and cognitive processes. Keywords: primate; encephalization; cognition; neocortex; brain. Introduction humans have been observed in several of the larger-brained species in different animal classes. Brain size shows a strong positive relationship Over the past three million years, endocranial with body size over a large set of animal species, volume has increased dramatically in our hominin but some species clearly have brains that are lineage, as has evidence for key cognitive much larger than expected, given their body size. innovations like biface tool manufacture and fire. Behaviors that would be considered intelligent in Taken together, these three observations have given rise to the idea that something about brain *Corresponding author. enlargement, once the allometric effects of Tel.: 1-514-398-6457; Fax: 1-514-398-65069 growth have been removed, has coevolved with E-mail: [email protected] cognition. Encephalization is the concept born DOI: 10.1016/B978-0-444-53860-4.00019-2 393 Author's personal copy 394 from this idea. In comparative biology, the con- allometric growth component in brain size cept describes the difference between animals in necessary? the amount of neuronal mass or volume available These questions are particularly relevant in beyond some value predicted by body size. In primates because recent work in neuroanatomy paleontology, it refers to increases in brain size (Burish et al., 2010; Gabi et al., 2010; observed over evolutionary time in some taxa. Herculano-Houzel et al., 2007) and comparative In neuroanatomy, it describes the relative psychology (Deaner et al., 2007) on this order increase in size of higher brain structures like has raised the possibility that absolute measures the neocortex1 and pallium in classes like birds of brain and neocortex size may be more relevant and mammals compared to the subpallium in than the traditional relative ones. Second, it is in more “primitive” clades like fish or reptiles. primates that we have the widest array of opera- Anatomical traits like folding are also sometimes tional measures of cognition on a wide sample described in similar terms, with highly gyrified of species, as well as attempts to synthesize this brains seen as more encephalized than smoother array into a common framework based on general ones. In this chapter, I first discuss some of the intelligence (Deaner et al., 2006; Reader et al., controversies surrounding the concept of 2011). The operational measures go from learning encephalization, and then review 26 large-scale and problem-solving tests in captivity (Johnson comparative studies that identify key correlates et al., 2002; Riddell and Corl, 1977) to taxonomic of primate encephalization. counts of tactical deception (Byrne and Whiten, 1990), innovation, tool use, social learning (Reader and Laland, 2002), and extractive forag- Problems with the concept of encephalization ing (Reader et al., 2011) taken mostly from the wild. The literature on primates also includes Encephalization is not a simple descriptive con- good quantitative estimates of lifestyles where cept. It is a relative one that implies a comparison complex cognition might provide a selective over time, a comparison between taxa, and/or a advantage, for example, group living (Dunbar, comparison of some neural structures (so-called 1998; Dunbar and Shultz, 2007a) or foraging for higher) with other neural structures (so-called high-quality dispersed food such as fruit (Barton, lower) or with the whole body. It is also a concept 1996; Clutton-Brock and Harvey, 1980; Fish and that mixes two levels of explanation: the struc- Lockwood, 2003). Third, primates feature one of tural level of neurons, brains, and bodies and the best neuroanatomical databases (Isler et al., the functional level of information processing. 2008; Stephan et al., 1981) to test structural Finally, as evidenced by the terms used in the first (Barton and Harvey, 2000; Clark et al., 2001; paragraph—“something about brain enlargement Finlay et al., 2001; Yopak et al., 2010) and func- has coevolved with cognition”—the concept can tional (Barton, 1998; Dunbar, 1998; Reader be fuzzy. What exactly is the “something” in the et al., 2011) hypotheses. brain that has coevolved with “cognition”?In Based on current knowledge, the structural fact, what is “cognition” and how can it be basis of encephalization in primates can be operationalized? Is the relative nature of described as a series of nested scaling encephalization justified and the removal of the relationships that link numbers of neurons and glia, cortical white and gray matter mass, neocor- tical volume, whole brain, body, and spinal cord 1I use the term “neocortex” throughout, rather than “iso- mass. The current consensus is that (1) numbers cortex,” because it is the term most often used in the literature of neurons and glia scale isometrically (i.e., 1 to I cover here. 1) with neocortex mass as well as brain mass, Author's personal copy 395 but cell density does not (Gabi et al., 2010; neurotransmitter metabolization, as well as blood Herculano-Houzel et al., 2007); (2) neocortical oxygenation changes that follow neuronal activ- gray matter scales isometrically (and white matter ity. The mapping of blood oxygenation changes with positive allometry, i.e., slope>1) with mass that is achieved with magnetic resonance imaging of the rest of the brain (Barton and Harvey, goes some way toward linking neuronal events 2000); (3) frontal white and gray matter have a with the identification of brain areas involved in positive allometric relationship with neocortical particular cognitive activities, but they do not mass (Smaers et al., 2010), as does (4) brain mass answer the question of size differences between with spinal cord neuron numbers (Burish et al., brain areas across different species. For example, 2010); (5) brain mass has a negative allometric a consensus seems to be developing among relationship (i.e., slope<1) with body mass, with neuroscientists (Deary et al., 2010) that tasks with different slopes at different taxonomic levels strong loadings on general intelligence involve a (Isler et al., 2008; Pagel and Harvey, 1989). In distributed network of at least 14 brain areas in primates (but not in other mammalian orders: humans (review by Jung and Hier, 2007 of 37 rodents: Herculano-Houzel et al., 2006; imaging studies involving 1557 subjects). Whether insectivores: Sarko et al., 2009), brain mass is the activation of the 14 areas during general intel- therefore equivalent to neuron numbers due to ligence tasks translates into more neurons, and the 1 to 1 relationship between them and the lack thus, a greater volume in each of the 14 brain of relationship of mass and volume